This invention concerns a speed control for a rotary machine, such as an electric motor, a turbine, or an internal combustion machine, in which for example a tachogenerator driven by the machine provides a signal of a frequency proportional to rotary speed, which is then processed so as to provide an output which can be used for speeding up or slowing down the machine whenever it slightly deviates from the desired speed. Buttafava discloses in U.S. Pat. No. 3,740,633 a frequency to voltage converter having a frequency error signal generator providing pulses, a constant current generator, a capacitor and two switches controlled by the pulses. One of the switches discharges the capacitor within a fixed time interval and the other switch connects the capacitor to the current generator for a time interval determined by the pulses.
From DE -PS No. 1 254 170, an electrical speed control system is known for controlling the speed of a turbine. A tachogenerator is coupled to the turbine. Its output frequency is multiplied and then modulated in a ring modulator by an alternating voltage of a prescribed frequency. This modulated wave, after passage through a low-pass filter is differentiated to obtain a train of pulses which periodically activate a transistor that discharges a capacitor that continuously receives a substantially constant charging current from a current source. If the differentiated pulses follow each other closely, the charge voltage of the capacitor reaches only low values. When pulses follow each other at greater intervals the charge voltage of the condensor reaches higher values. This charge voltage is sensed with a threshold switch, the output voltage of which controls the speed regulation process, for example, by controlling a valve that opens or closes the steam supply to the turbine.
In this known system, there is accordingly involved a two-point regulation system, which is unsatisfactory for many purposes that require speed regulation. Furthermore, particular precautions must be taken in order to prevent the machine on running up to speed from running right through the regulation range and consequently continuing to increase in speed out of control. For this purpose, the known system requires a special protection circuit.